Researchers from University College London studied the brains of 105 people - 80 of whom were bilingual.

They found learning other languages altered grey matter - the area of the brain which processes information - in the same way exercise builds muscles.

People who learned a second language at a younger age were also more likely to have more advanced grey matter than those who learned later, the team said.

Scientists already know the brain has the ability to change its structure as a result of stimulation - an effect known as plasticity - but this research demonstrates how learning languages develops it.

It means that older learners won't be as fluent as people who learned earlier in life

Andrea Mechelli, of University College London

The team took scans of 25 Britons who did not speak a second language, 25 people who had learned another European language before the age of five and 33 bilinguals who had learned a second language between 10 and 15 years old.

The scans revealed the density of the grey matter in the left inferior parietal cortex of the brain was greater in bilinguals than in those without a second language.

The effect was particularly noticeable in the "early" bilinguals, the findings published in the journal Nature revealed.

The findings were also replicated in a study of 22 native Italian speakers who had learned English as a second language between the ages of two and 34.

Lead researcher Andrea Mechelli, of the Institute of Neurology at UCL, said the findings explained why younger people found it easier to learn second languages.

Impact

"It means that older learners won't be as fluent as people who learned earlier in life.

"They won't be as good as early bilinguals who learned, for example, before the age of five or before the age of 10."

But Cilt, the national centre for languages, cast doubt on whether learning languages was easier at a younger age.

A spokeswoman said: "There are conflicting views about the comparative impact of language learning in different age groups, based both on findings and anecdotal evidence."

However, she said it was important to get young people learning languages in the UK.

Only one in 10 UK workers can speak a foreign language, a recent survey revealed.

But by 2010 all primary schools will have to provide language lessons for children.